A novel, endogenous opioid-like peptide, orphanin FQ/nociceptin (OFQ/N), and its receptor, ORL-1, have been isolated from brain. OFQ/N and ORL-1 protein and mRNA are widely expressed in the central nervous system and periphery. OFQ/N has a complex pharmacology and has been shown to be either analgesic or hyperalgesic in rodent assays of cutaneous pain. The role of OFQ/N in visceral pain is unknown. Our preliminary studies show that OFQ/N, like morphine or other opioid compounds, significantly inhibits colonic transit in vivo and stimulates colonic smooth muscle in vitro. More important, OFQ/N's actions are insensitive to opioid receptor antagonists such as naloxone. Thus, OFQ/N and its receptor ORL-1 may be novel therapeutic targets for the treatment of visceral pain which would lack the abuse and addiction liabilities of opioids such as morphine or fentanyl. The overall objective of this proposal is to evaluate the role of OFQ/N in visceral pain and colonic motility. Our hypothesis is that OFQ/N is an endogenous principal modulator of these processes. Utilizing both human and murine species, we propose to test this hypothesis through the following specific aims: (1) operationally identify the location(s) within the organism where OFQ/N acts to regulate colonic activity and evaluate the interactions between OFQ/N and opioids in vivo; correlate the functional studies with structural studies which will localize the expression of OFQ/N and its receptor ORL-1 mRNA and polypeptide in murine and human colon; (2) because 5-HT has been implicated as an important neurotransmitter involved in the control of GI motility and visceral sensation, we will examine 5-HT's involvement in the mechanism of action of OFQ/N on colonic function in human and murine tissue, (3) we will pioneer the use of the mouse in an assay of visceral pain and use this assay to evaluate the role of OFQ/N in modulating visceral pain by utilizing ORL-1 receptor knockout animals in this novel visceral pain assay. The proposed studies will contribute new and clinically relevant data on a novel endogenous opioid which could lead to the development of analgesics with reduced GI side effects.